1. Field of the Invention
The present invention relates to video surveillance, radio transmitters and regulatory schemes for controlling their use, and to methods and apparatus for efficiently and economically transmitting high resolution video signals using a Frequency Hopped, Spread Spectrum (FHSS) radio transmitter.
2. Discussion of the Prior Art
Ensuring security for large facilities can be difficult, since labor costs usually compel choosing fewer security personnel than might otherwise seem ideal. Large facilities now rely on increasingly complex video surveillance systems in efforts to provide greater security everywhere, all the time.
Installation costs become prohibitive, however, since traditional video cameras must be installed with a compliment of wires to carry video signals from the camera to a central monitoring location and those wires must be installed in a manner which meets applicable building and safety codes. In order to avoid high installation costs for wired video surveillance systems, installers have sought video cameras which can be installed with wireless video signal telemetry links; traditionally, such links are either analog or digital radio links.
A number of traditional analog television signal transmission systems have been used with great success around the world. In the United States, the National Television Standards Committee (NTSC) provided a standard video signal format for composite video signal transmission. The NTSC standard provides for a refresh rate of 60 half frames (interlaced) per second, or 30 completed frames per second. Each NTSC frame contains 525 lines which can represent up to 60,000,000 different colors. The NTSC standard is incompatible with most computer video standards which generally use computer specific video signal processing formats, such as the Red-Green-Blue (RGB) video signal processing format. An NTSC compatible signal is an analog video signal which includes horizontal synchronization (sync) and vertical sync timing signals incorporated along with the video information for each frame to be displayed on the viewers screen.
In other parts of the world, other standards have been adopted for analog television or video signal transmission, for example, in Europe, a standard identified by the acronym PAL provides 635 lines per frame at 25 frames per second. Other well known television standards used elsewhere in the world are identified by the following acronyms or terms: PAL-M, SECAM, SECAM-M, D-MAC, PALplus and HiVision. These standards for analog television or video signal transmission are well documented in technical specifications widely circulated among those having skill in the art.
While analog video or television signal transmission is well known for commercial uses and is perfectly suitable for those video surveillance system applications where wires or cables can be used to pass the video data from a camera to a central location, analog video transmission technology is ill suited to use in a video surveillance system having cameras connected via wireless links. The Federal Communications Commission (FCC) requires each television signal transmitter to be licensed, and it would be nearly impossible to acquire the appropriate licenses to permit, for example, a dozen high resolution television cameras to communicate simultaneously with a central monitoring location. This would be analogous to obtaining FCC licenses for every television station in a hypothetical major city in the United States.
FCC Regulators and spectrum resource managers have been confronted with an increasingly crowded electromagnetic spectrum because users of increasingly varied technologies incorporate wireless radio links into devices which were previously tethered by wires for passing video signals, audio signals, data telemetry or the like.
In response, special license-free bands have been designated by spectrum management agencies around the world for users of low power wireless data telemetry radios to operate wireless links. For example, in the United States, the FCC has designated license-free bandwidth segments of the radio frequency spectrum and made them available for industrial, scientific and medical (ISM) uses. In order to minimize problems with electromagnetic compatibility (EMC) between un-licensed radio transmitters and other radio systems, complex and rigorous regulations have been promulgated to control radiation of RF or microwave energy.
To cite a concrete example, referring to the Oct. 1, 1997 edition of Title 47 of the Code of Federal Regulations (47 C.F.R.), U.S. telecommunications regulations, such as 47 C.F.R. §15.245, §15.247 and others, limit maximum peak output power and electric field strength, as measured in units of volts (or millivolts) per meter. Section 15.249 provides that transmission within the ISM bands, 902–928 MHZ, 2400–2483.5 MHZ and 5725–5875 MHZ shall be limited in electric field strength to 50 millivolts per meter at the fundamental frequency, and at 24.0–24.25 GHz shall be limited to 250 millivolts per meter at the fundamental frequency. There are also strict bandwidth limitations imposed on unlicenced users of the defined channels included in this spectrum.
Distribution of video information as part of a surveillance network environment presents certain challenges for the network designer. For example, with the increasing popularity of multimedia applications, modern computer equipment standards increasingly require digitally encoded visual data. Digital images are, by nature of their graphical content, relatively more complex than other signal types such as digital audio and so require significant bandwidth within the communication channels to transport the complex information embodying the images. Accordingly, to transport such information efficiently, digital imaging applications often rely on the use of data compression techniques to reduce the amount of information to be transmitted within the network to manageable levels.
In light of the above, it is not surprising that image data compression often involves reducing the amount of data required to represent a digital image. One common basis of the reduction process is the removal of “redundant” data. In addition, inherent non-linearities in human visual perception can be leveraged to reduce the amount of data to be displayed in succeeding frames of a motion video. Accordingly, existing compression schemes exploit correlation in both space and time for video signals. Spatial compression is known as intra-frame compression, while temporal compression is known as inter-frame compression. Video surveillance systems using these compression technologies are expensive to manufacture and present problems with image quality and image transit time.
Generally, methods that achieve high compression ratios (e.g., over 50:1 ) are lossy, in that the data that is reconstructed from a compressed image is not identical to the original. The “losses” experienced in the compression process are manifested as distortions in the reconstructed images. While lossless compression methods do exist, their compression ratios are far lower. For most commercial, industrial and consumer applications, lossy methods are preferred because they save on required storage space and communication channel bandwidth.
Various techniques have been adopted as industry standards for motion image compression, including Recommendation H.261 of the Consultative Committee on International Telephony and Telegraphy (CCITT) for video conferencing, and schemes proposed by the Moving Pictures Expert Group (MPEG) for full-motion compression for digital storage medium. While such video compression methods can compress data at high ratios with acceptable quality in the decompressed images, they do not necessarily provide high data compression ratios for use in limited bandwidth environments such as would be needed for use in the ISM bands. Further, these prior compression processes do not include means for correcting distortions that may be present in earlier-transmitted frames. For example, in those prior video compression schemes that attempt to improve compression efficiency by reducing inter-frame redundancy with the use of “motion estimation” and/or “motion prediction”, earlier-transmitted frames are updated by compressing and transmitting the difference between a current frame and a preceding frame. In this manner, the compression process is made more efficient, as subsequent frames do not need to be compressed in their entirety if the extent of the changes between frames is limited. Although these schemes tend to conserve bandwidth, it is likely that distortions will be present in the earlier-transmitted frames, and those distortions are necessarily carried through to subsequent frames. With each new frame, additional compression distortions are introduced into the reconstructed images, and so compression distortions tend to accumulate from frame to frame. Prior art compression schemes do not provide means to reduce or eliminate these distortions, unless the transmitted frame rate is kept high enough to reduce accumulated compression distortion to an acceptable level. Consequently, even using systems incorporating the relatively expensive industry standard data compression methods, too much bandwidth is likely to be needed if acceptable image quality is to be obtained in a wireless video surveillance system.
Others have utilized differing approaches to transmit video images; for example, U.S. Pat. No. 5,859,664, to Dent, teaches use of a transmission method in which analog composite video signals are first stripped of sync signal components which are replaced by frequency hopping codes and then modulated for transmission over a frequency hopped channel; at the receive end, the signal is demodulated and a composite video signal is synthesized in a powerful processor by inserting synthesized video sync and line sync signals. While the Dent system does sidestep the expense and poor quality of video transmission systems using the above described compression technologies; a cumbersome and expensive process of replacing sync signals with other signals and then re-constituting the composite video signal is used in their place. The Dent processor must re-create sync components of the composite video signal before the video image can be displayed on a conventional monitor and so video signal transit time may also be adversely affected unless expensive and very fast processing circuitry is employed.
Video surveillance equipment used in security systems must also be secure and robust. Ideally, any wireless data telemetry system used as part of a security system must be resistant to surreptitious eavesdropping and must be resistant to intentional signal interference or jamming. Traditional AM or FM transmission systems generate signals which are all too easy to locate, intercept and jam.
There is a need, therefore, for an inexpensive, robust, secure wireless video image transmission system and method which will provide high resolution images in a legally compliant segment of bandwidth.